Certain components of nuclear reactors, such as heat exchangers, cluster guide pins, pipework, fasteners for fastening components made of steel and used for making the cooling circuits of light water nuclear reactors or of nuclear reactors having a heat-conveying fluid in the form of a gas or a molten salt or a liquid metal, are made out of nickel-based alloys, e.g., out of various types of Inconel®. At high temperature and at high pressure, such components need to present good resistance to oxidation, to corrosion, to creep, and to cyclical stresses both thermal and mechanical, and they need to do so for long periods of time (several tens of years), and nickel-based alloys are well adapted to such purposes.
Fuel assemblies for light water nuclear reactors may also have some of their structural components made of a nickel-based alloy, with the 718 alloy being a preferred example. This applies in particular to grid springs that are usually fabricated from strips of such alloys, and hold-down springs made either from flat half-finished products for spring blades, or from wires for coil springs, and also fastener elements, that are made from bars.
The nickel-based alloys that can be used in these contexts have the following general composition, expressed in percentages by weight: C≦0.10%; Mn≦0.5%; Si≦0.5%; P≦0.015%; S≦0.015%; Ni≧40%; Cr=12%-40%; Co≦10%; Al≦5%; Mo=0.1%-15%; Ti≦5%; B≦0.01%; Cu≦5%; W=0.1%-15%; Nb=0-10%; Ta≦10%; the balance being Fe and inevitable impurities that result from processing. Those elements for which a minimum value is not given may be completely absent, or present solely as traces. There may also be small amounts of other elements that are used more rarely, for the purpose of adjusting certain chemical or mechanical properties, and that do not radically change the behavior of the alloy in terms of its sensitivity to environmentally-assisted cracking, which in an aqueous medium gives rise to a phenomenon of corrosion under stress.
Typically, the composition of 718 alloy, a particular example of such alloys, is as follows: C≦0.08%; Mn≦0.35%; Si≦0.35%; P≦0.015%; S≦0.015%; Ni=50%-55%; Cr=17%-21%; Co≦1%; Al=0.2%-0.8%; Mo=2.8%-3.3%; Ti=0.65%-1.15%; B≦0.006%; Cu≦0.3%; Nb+Ta=4.75%-5.5%; the balance being Fe and inevitable impurities that result from processing. It may also contain a few hundreds of parts per million (ppm) of Mg.
A problem that is of increasing importance in the operation of reactors containing such components is the ability of the components to withstand environmentally-assisted cracking. Firstly, it is desirable to lengthen as much as possible the durations of operating cycles for fuel assemblies. Thus, it is desirable to lengthen the present usual duration of 12 months to 18 months or even 24 months. Secondly, the conditions specific to the primary medium in light water reactors (LWR) are favorable to the development of environmentally-assisted cracking. The same applies to reactors in which the heat-conveying fluid is gas or molten salt or liquid metal, given the very high temperatures that are reached, which exacerbate oxidation phenomena. Experience with pressurized water reactors has shown in particular that grid springs made of 718 alloy can fracture while in use as a result of a process of environmentally-assisted cracking, specifically stress corrosion cracking (SCC). Fractures or cracks have also been found in cluster guide pins made of X750 alloy, in the pipework of steam generators made of 600 alloy, in the bottom-of-vessel bushings, and in welded zones, all of these parts being made of various grades of nickel-based alloy.
To improve the reliability of nickel-based alloy components, in particular components of 718 alloy, it is therefore necessary to find means for reducing the sensitivity of such components to environmentally-assisted cracking.
Until now, the solutions used have, above all, involved good industrial practice or palliative measure.
Thus, proposals have been made to modify the surface state of a structural element either mechanically (shot blasting, microbeading, sand blasting, . . . ) or chemically (electro-polishing). For example, document JP-A-2000 053 492 teaches removing the outermost surface layer of a monocrystalline cast material of Ni-based superalloy by oxidizing the layer and then by performing electrochemical polishing. After that, heat treatment at a temperature equal to or greater than the recrystallization temperature is performed. That eliminates residual surface stresses in the material that make it sensitive to environmentally-assisted cracking. The surface is then covered in a ceramic layer. That document teaches applying that method to gas turbine blades, however the modification to the surface state of the material for eliminating residual stress has also been performed on the tubes of steam generators that are made of 600 and 690 alloys.
Another method consists of applying a suitable coating on the materials. Thus, it is common to nickel-plate 718 alloy grid springs in order to reduce the number of spring fractures in service. Other types of coatings, e.g. surface treatment by diffusion, are also possible. Thus, document U.S. Pat. No. 5,164,270 proposes implanting Nb and/or Zr in the surface of a ferrous alloy having 9% to 30% Cr, and exposing it to a gaseous mixture of O2 and S. That could also be applied to an Ni-based alloy.
Another solution consists in performing overall or local heat treatment at high temperature (1100° C.) on the structural elements, leading to changes in the microstructure of the material. Local treatment is thus performed on the bends of 600 alloy steam generators. Attempts have also made in that way to eliminate all traces of δ phase in 718 alloy (see document U.S. Pat. No. 5,047,093).
Another solution consists of modifying the chemical composition of the material in more or less radical manner, which can sometimes lead to developing new alloy grades. Thus, 600 alloy has been replaced by 690 alloy in the manufacture of steam generator tubes. That approach is expensive in research and development time, and it does not always lead to results that are technically and/or economically viable for industrial applications.
Finally, action has been taken not on the materials themselves, but on the design of the structures, seeking to reduce the stress levels to which they are subjected. That approach is likewise in any event expensive in development time and often leads to failure.